Some laser projectors for projecting and displaying an image based on the raster scanning of a laser beam have a high operating rate in the range from several tens kHz to 100 kHz for horizontally scanning and a high deflection angle represented by an optical angle of ±20° or greater. One approach for performing a scanning process with such a high operating rate and have a high deflection angle is to use a vibratory mirror which operates near a resonant point thereof.
The vibratory mirror can have a high deflection angle in a small operating frequency range near the resonant frequency thereof. However, if the operating frequency deviates from the resonant frequency, then the deflection angle of the vibratory mirror is extremely reduced. The resonant frequency depends on the material, shape, temperature, etc. of the vibratory mirror and tends to vary due to manufacturing variations and the operating temperature. To operate the vibratory mirror through a high deflection angle, therefore, it is necessary to keep the resonant frequency and the operating frequency as close to each other as possible. The vibratory mirror is thus adjusted to either bring the resonant frequency thereof close to the operating frequency or bring the operating frequency close to the resonant frequency thereof.
As the deflection angle becomes greater, the vibratory mirror is more likely to fail to operate. When the vibratory mirror is to operate through a high deflection angle, it is desirable to control the vibratory mirror such that its deflection angle will not exceed a prescribed deflection angle. When the vibratory mirror operates with its operating frequency deviating from the resonant frequency, then since the intensity of an input signal needs to be higher than when the vibratory mirror operates at the resonant frequency, a circuit for actuating the vibratory mirror undergoes an increased load.
In order to adjust the deflection angle to operate the vibratory mirror through a high deflection angle, it is desirable that the deflection angle of the vibratory mirror not exceed a predetermined deflection angle and that the vibratory mirror operate at the resonant frequency.
In order to adjust the deflection angle of the vibratory mirror, it is necessary to accurately detect the deflection angle of the vibratory mirror. According to a widely performed process of detecting the deflection angle of the vibratory mirror, a light sensor is used to detect the deflection angle of the vibratory mirror based on the timing at which a light beam passes through the light sensor.
For example, JP-A No. 2004-053943 (Patent document 1) discloses a technique wherein synchronism sensors are disposed respectively at the start and end points of the scanning stroke of the laser beam for detecting the deflection angle of a vibratory mirror, and the drive frequency of the vibratory mirror is corrected based on the detected results. A process of detecting and adjusting the deflection angle of the vibratory mirror as disclosed in Patent document 1 will be described below with reference to FIG. 1. FIG. 1(a) is a block diagram of a system for controlling a semiconductor laser and a movable mirror as disclosed in Patent document 1, and FIG. 1(b) is a timing chart of the amplitude of the movable mirror and drive pulses.
In FIG. 1(a), the reference numeral 901 represents drive pulse generator, 902 the movable mirror driver, 904 the synchronism detecting sensor, 905 the end detecting sensor, 906 the LD driver, 907 the clock pulse generator, 908 the phase synchronizer, 909 the magnification calculator, and 910 the amplitude calculator.
Drive pulse generator 901 frequency-divides a reference clock with a programmable frequency divider (not shown), to generate a pulse train (T<T0/4) having a frequency which is twice drive frequency fd (=1/T0) and a duty ratio of 50% or lower such that voltage pulses will be applied, one in a ½ period of a movable mirror, only during a period from a maximum amplitude level to a horizontal orientation, as shown in FIG. 1(b). Drive pulse generator 901 then delays the pulse train by phase delay δ with a PLL circuit, and applies the pulse train at drive frequency fd to movable mirror driver 902.
When the system is turned on or when the system is activated from a standby mode, the frequency-dividing ratio is continuously changed by the programmable frequency divider to vary drive frequency fd from a high frequency value and to drive the movable mirror at varying drive frequency fd. When the scanning angle is increased until the light beam is detected by synchronism detecting sensor 904, the system judges that the movable mirror operates in a resonant vibration band. At the same time, the system calculates a scanning angle based on the time difference between the start and end points of the scanning stroke, and sets the drive frequency such that the deflection angle (amplitude) of the movable mirror will be of a predetermined angle.
Japanese utility model No. 2524140 (Patent document 2) discloses a laser beam scanning device for scanning a laser beam wherein a scanning mirror is irradiated with an irradiating light beam at an incident angle which is different from the laser beam, and a line sensor is provided as photodetector means for detecting the irradiating light beam reflected from the scanning mirror. The line sensor detects the deflection angle of the scanning mirror, and the laser beam scanning device corrects the deflection angle of the scanning mirror beyond standard deviation.
FIG. 2 is a perspective view of the laser beam scanning device disclosed in Patent document 2. A process of detecting the deflection angle of the vibratory mirror as disclosed in Patent document 2 will be described below with reference to FIG. 2.
As shown in FIG. 2, main laser beam 210 from a laser oscillator (Nd:YAG laser oscillator or the like, not shown) is focused onto specimen surface 205 by mirror 203, scanning mirror 202 of galvanometer-type optical scanner (hereinafter referred to as “scanner”) 201, and fθ lens 204, and is positioned by scanning mirror 202 as it is turned.
Main laser beam 210 is applied to scanning mirror 202 at an incident angle of 45° and folded by scanning mirror 202 through 95°, i.e., directed to fθ lens 204 at an exit angle of 135°.
Laser beam 211 emitted from semiconductor laser 206 is converted by collimator lens 207 into a parallel beam, which is applied to scanning mirror 202. The parallel beam is applied to scanning mirror 202 at an incident angle different from main laser beam 210, and is reflected from scanning mirror 202 at an angle different from the exist angle of main laser beam 210. The reflected laser beam is focused onto line sensor 209 by cylindrical lens 208.
When the deflection angle having a maximum amplitude of scanner 201 is changed due to the heating of scanner 201 itself during its operation, the focused position on line sensor 209 of laser beam 211 emitted from semiconductor laser 206 and reflected by scanning mirror 202 is also changed, resulting in a change in the positional information from line sensor 209. A signal for driving scanner 201 is corrected based on the positional information to position main laser beam 210 accurately.
Patent document 3 (JP-A No. 2005-241482) discloses a method of detecting the resonant frequency of a vibratory mirror mounted in a laser display device.
FIG. 3 is a perspective view schematically showing the laser display device disclosed in Patent document 3. The method of detecting the resonant frequency of the vibratory mirror mounted in the laser display device disclosed in Patent document 3 will be described below with reference to FIG. 3.
FIG. 3 shows photodetector (line sensor) 301, light source 401, deflecting means 402, emitted light beam 403, deflected light beams 404, 405, scanning line 410, second deflecting means 411, deflected light beam 412, shield 413, scanning area 414, scanning line path 415, and projection plane 420. In the laser display device shown in FIG. 3, emitted light beam 403 from light source 401, which is a laser, is two-dimensionally deflected by deflecting means 402 and second deflecting means 411 to display an image on projection plane 420.
FIG. 4(a) is a schematic diagram showing the manner in which the deflected light beam moves back and forth on photodetector 301 shown in FIG. 3, and FIG. 4(b) is a schematic diagram showing the manner in which the deflected light beam is deflected through a different deflection angle. As shown in FIG. 4, photodetector 301 as a line sensor is divided into a plurality of light detecting areas 302 that are successively numbered from the leftmost light detecting area “1” rightwardly up to “N”. Reference numeral 303 represents an area scanned by the deflected light beam.
When the power supply of the laser display device is turned on, it starts a control process to fully energize light source 401. Then, the frequency of a signal applied to deflecting means 402 is set to a preset startup frequency, and the folding position of the deflected light beam at the frequency is detected by photodetector 301. Thereafter, the frequency is changed by preset steps. The frequency is changed and the folding position is detected repeatedly until a preset end frequency is reached.
It is assumed that when the folding position of the deflected light beam is in the leftmost light detecting area, then the folding position is detected as “1” (see FIG. 4(a)), and the folding position of the deflected light beam is in a pth area counted from the leftmost light detecting area, then the folding position is detected as “p” (see FIG. 4(p)). When the folding position goes beyond photodetector 301, i.e., when it is positioned rightwardly of photodetector 301, the folding position is detected as “N+1”. Conversely, when the folding position falls short of photodetector 301, the folding position is detected as “0”. The deflecting state of deflecting means 402 can be detected by thus detecting the folding positions on and near photodetector 301.
The relationship between frequencies of the signal applied to photodetector 402 and folding positions of the deflected light beam detected by photodetector 301 is shown as a graph in FIG. 5. Plotted points in the graph are in a substantially axisymmetric pattern, and the frequency at the axis of symmetry is calculated as resonant frequency fc. The frequency of a signal to be applied to photodetector 402 is determined based on the calculated frequency.    Patent document 1: JP-A No. 2004-053943    Patent document 2: Japanese utility model No. 2524140    Patent document 3: JP-A No. 2005-241482